1. Field of the Invention
The invention relates generally to the field of differential amplifier circuits, and specifically provides a dual differential amplifier circuit that is useful in shaping waveforms of electrical signals.
2. Background
In a digital data processing system, mass storage devices record digital data on, typically, magnetic storage media which moves relative to a transducer or read/write head. The digital data is represented as a series of transitions in the magnetic field recorded on the media as it moves relative to the read/write head. An electrical current is applied to the head, which generates a magnetic field in response thereto. The direction of the magnetic field varies in response to the applied electrical signal, which, in turn, varies in response to the data to be written. Accordingly, the magnetic field written on the media corresponds to the digital data.
After the magnetic signal is recorded on the media, the read/write head can read the signal and provide an electrical signal which can be used to determine the stored data. In this process, the read/write head is placed adjacent to the portion of the media in which the data was previously written. The media moves relative to the read/write head so that the previously recorded magnetic field generates an electrical signal. Specifically, the transitions in the recorded magnetic field cause variations in the electrical signal, which, in turn, are processed to recover the recorded digital data.
The electrical signal from a typical read/write head is very small. That is, the peak-to-peak voltage of the signal, which is a measure of the voltage difference of the signal between the positive peak and the negative peak, or valley, in the signal that is received from a read/write head in a typical mass-storage device is not very large, and it is difficult to distinguish from the background noise which is present in any system. Furthermore, as data is recorded on media at higher recording densities, typically the electrical signal from a read/write head is reduced and the noise increased.
The timing of a peak or valley in a recorded signal relates to the passage of the transducer over the location of a transition in the magnetic field recorded on the disk. It is used not only in determining the stored data but also in providing several timing signals which are used to determine the stored data, particularly difficult. As the recording densities increase, however, the timings between peaks and valleys concomitantly decrease, and the transitions become closer together. As a result, the read/write head is affected not only by the magnetic fields defining the transition proximate the head, but also by magnetic fields defining adjacent transitions, resulting in "peak shift" or "intersymbol interference" between adjacent the positive and negative peaks. The presence of peak shift in a read signal increases the difficulty of precisely identifying the timings of the peaks and valleys in the signal received from a read/write head. Accordingly, complicated electrical equalizing circuit such as cosine filters have been developed as pulse slimmers to reduce the width of the read pulse and help locate the peaks and valleys of the read signal.
Differential amplifiers are often used in a number of applications, including data recovery in a mass storage device, to provide enhanced sensitivity and reduction in noise. In a conventional differential amplifier, the output signal is essentially the result of a comparison between two input signals from a signal source; that is, the output signal is related to the difference between the input signals from the signal source. Since normally input signals from a single source will have common noise patterns, differencing the signals cancels out at least the common noise. The elimination of the noise common to the two input signals further enhances the usable sensitivity of the circuit to the desired input signal. A typical differential amplifier, however, does not assist in processing the signal to identify the peaks and valleys therein.